When at least one of the gear axes rotates relative to the frame in addition to the gear's own rotation about its own axes, the train is called a planetary gear train or epicyclic gear train. The term ``epicyclic'' comes from the fact that points on gears with moving axes of rotation describe epicyclic paths. When a generating circle (planet gear) rolls on the outside of another circle, called a directing circle (sun gear), each point on the generating circle describes an epicycloid, as shown in Fig. 2.7.
Generally, the more planet gears there are, the greater is the torque capacity of the system. For better load balancing, new designs have two sun gears and up to 12 planetary assemblies in one casing.
In the case of simple and compound gears, it is not difficult to visualize the motion of the gears, and the determination of the speed ratio is relatively easy. In the case of epicyclic gear trains, it is often diffuclt to visualize the motion of the gears. A systematic procedure using the contour method is presented in what follows. The contour method is applied to determine the distribution of velocities for an epicyclic gear train.
Mechanical Engineer's Handbook
Edited by
Dan B. Marghitu
Department of Mechanical Engineering, Auburn University,
Auburn, Alabama
Academic Press Series in Engineering
Series Editor
J. David Irwin
Auburn University
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